feat: an album holding a song twice beside its single keeps one copy (#5401)

A cross-release group was only ever linked, never merged, so an album that
held the song twice among the releases kept both: Humanz has Saturnz Barz,
Andromeda, We Got the Power and Garage Palace as its FLAC and a YouTube mp3,
each beside the single. The resolver now merges an album's extra copies
into its best (the one that is not a video rip, else ProposeSurvivor:
lossless first) with ReplaceTrack, history and all, then links what is
left as one song.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/song_link_test.go

@@ -2,6 +2,8 @@ package library
 
 import (
 	"context"
+	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/jackc/pgx/v5/pgtype"
@@ -218,3 +220,68 @@ func songKeys(t *testing.T, q *dbq.Queries, ids ...pgtype.UUID) []pgtype.UUID {
 	}
 	return out
 }
+
+// A cross-release group whose album holds the song twice (Humanz: Saturnz
+// Barz as a FLAC and a YouTube mp3, beside the single, #5401): the album's
+// extra copy is merged into its FLAC, the single stays, and the two copies
+// left are linked as one song.
+func TestResolveDuplicates_FoldsAnAlbumsExtraCopyInACrossReleaseGroup_Integration(t *testing.T) {
+	pool := newPool(t)
+	ctx := context.Background()
+	q := dbq.New(pool)
+	dir := t.TempDir()
+	artist, err := q.UpsertArtist(ctx, dbq.UpsertArtistParams{Name: "Gorillaz", SortName: "Gorillaz"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	track := func(album, name, format string) dbq.Track {
+		t.Helper()
+		a, err := q.UpsertAlbum(ctx, dbq.UpsertAlbumParams{Title: album, SortTitle: album, ArtistID: artist.ID})
+		if err != nil {
+			t.Fatal(err)
+		}
+		path := filepath.Join(dir, album, name)
+		if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(path, []byte("audio"), 0o644); err != nil {
+			t.Fatal(err)
+		}
+		tr, err := q.UpsertTrack(ctx, dbq.UpsertTrackParams{
+			Title: "Saturnz Barz", AlbumID: a.ID, ArtistID: artist.ID,
+			DurationMs: 181000, FilePath: path, FileSize: 100, FileFormat: format,
+		})
+		if err != nil {
+			t.Fatal(err)
+		}
+		return tr
+	}
+	flac := track("Humanz", "04 Saturnz Barz.flac", "flac")
+	mp3 := track("Humanz", "06 Saturnz Barz (feat. Popcaan).mp3", "mp3")
+	single := track("Saturnz Barz", "01 Saturnz Barz.mp3", "mp3")
+	var groupID pgtype.UUID
+	if err := pool.QueryRow(ctx,
+		`INSERT INTO duplicate_groups (member_key, tier) VALUES ('humanz-saturnz', 'exact') RETURNING id`,
+	).Scan(&groupID); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := pool.Exec(ctx, `INSERT INTO duplicate_group_members (group_id, track_id) VALUES ($1, $2), ($1, $3), ($1, $4)`,
+		groupID, flac.ID, mp3.ID, single.ID); err != nil {
+		t.Fatal(err)
+	}
+
+	res, err := ResolveDuplicates(ctx, pool, nil, "", true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if res.Merged != 1 || exists(mp3.FilePath) || !exists(flac.FilePath) || !exists(single.FilePath) {
+		t.Fatalf("result %+v, mp3 %v flac %v single %v; want only the album's mp3 merged away",
+			res, exists(mp3.FilePath), exists(flac.FilePath), exists(single.FilePath))
+	}
+	if _, err := q.GetTrackByID(ctx, mp3.ID); err == nil {
+		t.Error("the album's extra copy is still a track")
+	}
+	if res.SongsLinked != 1 {
+		t.Errorf("songs linked = %d, want the FLAC and the single linked as one song", res.SongsLinked)
+	}
+}